Recently, an optical information recording disc of DVD (Digital Video Disc) type has been paid an attention, because the information recording disc of DVD type can record a great amount of information in a limited space volume. The information recording discs of DVD type are classified into DVD-ROM (for read-out only), DVD-R (in which additional information is recordable), and DVD-RAM (in which a recorded information is rewritable).
The information recording disc of DVD type has a sandwich structure. In more detail, it comprises a pair of substrate discs having an information recorded layer from which the information is readable by applying thereto a laser beam or an information recordable layer in which an information is recordable by applying thereto a laser beam or comprises a substrate disc having an information recorded layer from which the information is readable by applying thereto a laser beam or an information recordable layer in which an information is recordable by applying thereto a laser beam and a disc plate. The pair of substrates or the set of substrate and disc plate is combined using an adhesive layer under the condition that the information recorded layer or information recordable layer is placed between them.
Nikkei Electronics, No. 630 (1995, published Nihon Keizai Shimbunsha, namely, Japan Economic Journal), Specific Edition for "Optical Disc", as well as Nikkei New Media, Specific Edition for "DVD" (1995, published Nihon Keizai Shimbunsha) illustrate a schematic structure of a representative optical information recording disc of DVD-R type, which is shown in FIG. 1 of Drawings attached to this specification.
The representative information recording disc of DVD-R type of FIG. 1 comprises a pair of substrate discs 1a, 1b having a pre-groove for tracking, information recordable layers 2a, 2b placed on the substrate discs 1a, 1b, respectively, laser beam reflective layers 3a, 3b placed on the information recordable layers 2a, 2b, respectively, protective layers 4a, 4b, placed on the reflective layers 3a, 3b, respectively, and an adhesive layer 5 combining the protective layer 4a and the protective layer 4b. This information recording disc is produced by combining a pair of substrate discs 1a, 1b which are provided thereon with the information recording layer 2a, 2b, the reflective layer 3a, 3b, and the protective layer 4a, 4b, using an adhesive material 5.
The present inventor has studied the optical information recording disc of DVD-type and has noticed that the information recording disc of DVD-type sometimes shows poor mechanical strength as well as poor storage stability. A further study of the inventor has revealed that the poor mechanical strength and poor storage stability are caused by partial peeling of the edges of the substrates from the assembled recording disc structure. The peeling is sometimes observed between the substrate and the protective layer. The recording layer generally comprises a dye and, possibly, a small amount of a binder, and therefore, it has low cohesion strength. Moreover, the bonding strength between the reflective layer which is generally made of metal and the protective layer which is generally made of organic resin is low. Accordingly, the substrate disc easily separates from the protective layer at their outer peripheral ends when the information disc encounters physical shock or is stored for a long period under severe surrounding conditions. Particularly, if the substrate disc has a fin or flash at its outer or inner peripheral end, the separation occurs more easily when the information disc is brought into contact with other information disc or any other materials.